About Zeek

What Is Zeek?

Zeek is a passive, open-source network traffic analyzer. Many operators use Zeek as a network security monitor (NSM) to support investigations of suspicious or malicious activity. Zeek also supports a wide range of traffic analysis tasks beyond the security domain, including performance measurement and troubleshooting.

The first benefit a new user derives from Zeek is the extensive set of logs describing network activity. These logs include not only a comprehensive record of every connection seen on the wire, but also application-layer transcripts. These include all HTTP sessions with their requested URIs, key headers, MIME types, and server responses; DNS requests with replies; SSL certificates; key content of SMTP sessions; and much more. By default, Zeek writes all this information into well-structured tab-separated or JSON log files suitable for post-processing with external software. Users can also choose to have external databases or SIEM products consume, store, process, and present the data for querying.

In addition to the logs, Zeek comes with built-in functionality for a range of analysis and detection tasks, including extracting files from HTTP sessions, detecting malware by interfacing to external registries, reporting vulnerable versions of software seen on the network, identifying popular web applications, detecting SSH brute-forcing, validating SSL certificate chains, and much more.

In addition to shipping such powerful functionality “out of the box,” Zeek is a fully customizable and extensible platform for traffic analysis. Zeek provides users a domain-specific, Turing-complete scripting language for expressing arbitrary analysis tasks. Think of the Zeek language as a “domain-specific Python” (or Perl): just like Python, the system comes with a large set of pre-built functionality (the “standard library”), yet users can also put Zeek to use in novel ways by writing custom code. Indeed, all of Zeek’s default analyses, including logging, are done via scripts; no specific analysis is hard-coded into the core of the system.

Zeek runs on commodity hardware and hence provides a low-cost alternative to expensive proprietary solutions. In many ways Zeek exceeds the capabilities of other network monitoring tools, which typically remain limited to a small set of hard-coded analysis tasks. Zeek is not a classic signature-based intrusion detection system (IDS); while it supports such standard functionality as well, Zeek’s scripting language facilitates a much broader spectrum of very different approaches to finding malicious activity. These include semantic misuse detection, anomaly detection, and behavioral analysis.

A large variety of sites deploy Zeek to protect their infrastructure, including many universities, research labs, supercomputing centers, open-science communities, major corporations, and government agencies. Zeek specifically targets high-speed, high-volume network monitoring, and an increasing number of sites are now using the system to monitor their 10GE networks, with some already moving on to 100GE links.

Zeek accommodates high-performance settings by supporting scalable load-balancing. Large sites typically run “Zeek Clusters” in which a high-speed front end load balancer distributes the traffic across an appropriate number of back end PCs, all running dedicated Zeek instances on their individual traffic slices. A central manager system coordinates the process, synchronizing state across the back ends and providing the operators with a central management interface for configuration and access to aggregated logs. Zeek’s integrated management framework, ZeekControl, supports such cluster setups out-of-the-box.

Zeek’s cluster features support single-system and multi-system setups. That’s part of Zeek’s scalability advantages. For example, administrators can scale Zeek within one system for as long as possible, and then transparently add more systems when necessary.

In brief, Zeek is optimized for interpreting network traffic and generating logs based on that traffic. It is not optimized for byte matching, and users seeking signature detection approaches would be better served by trying intrusion detection systems such as Suricata. Zeek is also not a protocol analyzer in the sense of Wireshark, seeking to depict every element of network traffic at the frame level, or a system for storing traffic in packet capture (PCAP) form. Rather, Zeek sits at the “happy medium” representing compact yet high fidelity network logs, generating better understanding of network traffic and usage.

Why Zeek?

Zeek offers many advantages for security and network teams who want to better understand how their infrastructure is being used.

Security teams generally depend upon four sorts of data sources when trying to detect and respond to suspicious and malicious activity. These include third party sources such as law enforcement, peers, and commercial or nonprofit threat intelligence organizations; network data; infrastructure and application data, including logs from cloud environments; and endpoint data. Zeek is primarily a platform for collecting and analyzing the second form of data – network data. All four are important elements of any security team’s program, however.

When looking at data derived from the network, there are four types of data available to analysts. As defined by the network security monitoring paradigm, these four data types are full content, transaction data, extracted content, and alert data. Using these data types, one can record traffic, summarize traffic, extract traffic (or perhaps more accurately, extract content in the form of files), and judge traffic, respectively.

It’s critical to collect and analyze the four types of network security monitoring data. The question becomes one of determining the best way to accomplish this goal. Thankfully, Zeek as a NSM platform enables collection of at least two, and in some ways three, of these data forms, namely transaction data, extracted content, and alert data.

Zeek is best known for its transaction data. By default, when run and told to watch a network interface, Zeek will generate a collection of compact, high-fidelity, richly-annotated set of transaction logs. These logs describe the protocols and activity seen on the wire, in a judgement-free, policy-neutral manner. This documentation will spend a considerable amount of time describing the most common Zeek log files such that readers will become comfortable with the format and learn to apply them to their environment.

Zeek can also easily carve files from network traffic, thanks to its file extraction capabilities. Analysts can then send those files to execution sandboxes or other file examination tools for additional investigation. Zeek has some capability to perform classical byte-centric intrusion detection, but that job is best suited for packages like the open source Snort or Suricata engines. Zeek has other capabilities however that are capable of providing judgements in the form of alerts, through its notice mechanism.

Zeek is not optimized for writing traffic to disk in the spirit of a full content data collection, and that task is best handled by software written to fulfill that requirement.

Beyond the forms of network data that Zeek can natively collect and generate, Zeek has advantages that appeared in the What Is Zeek? section. These include its built-in functionality for a range of analysis and detection tasks, and its status as a fully customizable and extensible platform for traffic analysis. Zeek is also attractive because of its ability to run on commodity hardware, giving users of all types the ability to at least try Zeek in a low-cost manner.


Zeek has a rich history stretching back to the 1990s. Vern Paxson designed and implemented the initial version in 1995 as a researcher at the Lawrence Berkeley National Laboratory (LBNL). The original software was called “Bro,” as an “Orwellian reminder that monitoring comes hand in hand with the potential for privacy violations”.

LBNL first deployed Zeek in 1996, and the USENIX Security Symposium published Vern’s original paper on Zeek in 1998, and awarded it the Best Paper Award that year He published a refined version of the paper in 1999 as Bro: A System for Detecting Network Intruders in Real-Time.

In 2003, the National Science Foundation (NSF) began supporting research and advanced development on Bro at the International Computer Science Institute (ICSI). (Vern still leads the ICSI Networking and Security group.)

Over the years, a growing team of ICSI researchers and students kept adding novel functions to Zeek, while LBNL continued its support with funding from the Department of Energy (DOE). Much of Zeek’s capabilities originate in academic research projects, with results often published at top-tier conferences. A key to Zeek’s success was the project’s ability to bridge the gap between academia and operations. This relationship helped ground research on Zeek in real-world challenges.

With a growing operational user community, the research-centric development model eventually became a bottleneck to the system’s evolution. Research grants did not support the more mundane parts of software development and maintenance. However, those elements were crucial for the end-user experience. As a result, deploying Zeek required overcoming a steep learning curve.

In 2010, NSF sought to address this challenge by awarding ICSI a grant from its Software Development for Cyberinfrastructure fund. The National Center for Supercomputing Applications (NCSA) joined the team as a core partner, and the Zeek project began to overhaul many of the user-visible parts of the system for the 2.0 release in 2012.

After Zeek 2.0, the project enjoyed tremendous growth in new deployments across a diverse range of settings, and the ongoing collaboration between ICSI (co-PI Robin Sommer) and NCSA (co-PI Adam Slagell) brought a number of important features. In 2012, Zeek added native IPv6 support, long before many enterprise networking monitoring tools. In 2013, NSF renewed its support with a second grant that established the Bro Center of Expertise at ICSI and NCSA, promoting Zeek as a comprehensive, low-cost security capability for research and education communities. To facilitate both debugging and education, try.zeek.org (formerly try.bro.org) was launched in 2014. This provided an interactive way for users to test a script with their own packet captures against a variety of Zeek versions and easily share sample code with others. For Zeek clusters and external communication, the Broker communication framework was added. Last, but not least, the Zeek package manager was created in 2016, funded by an additional grant from the Mozilla Foundation.

In the fall of 2018, the project leadership team decided to change the name of the software from Bro to Zeek. The leadership team desired a name that better reflected the values of the community while avoiding the negative connotations of so-called “bro culture” outside the computing world. The project released version 3.0 in the fall of 2019, the first release bearing the name Zeek. The year 2020 saw a renewed focus on community and growing the Zeek community, with increased interaction via social media, webinars, Slack channels, and related outreach efforts.

For a history of the project from 1995 to 2015, see Vern Paxson’s talk from BroCon 2015, Reflecting on Twenty Years of Bro.

For background on the decision to rename Bro to Zeek, see Vern Paxson’s talk from BroCon 2018, Renaming Bro.



At a very high level, Zeek is architecturally layered into two major components. Its event engine (or core) reduces the incoming packet stream into a series of higher-level events. These events reflect network activity in policy-neutral terms, i.e., they describe what has been seen, but not why, or whether it is significant.

For example, every HTTP request on the wire turns into a corresponding http_request event that carries with it the involved IP addresses and ports, the URI being requested, and the HTTP version in use. The event however does not convey any further interpretation, such as whether that URI corresponds to a known malware site.

The event engine component comprises a number of subcomponents, including in particular the packet processing pipeline consisting of: input sources, packet analysis, session analysis, and file analysis. Input sources ingest incoming network traffic from network interfaces. Packet analysis processes lower-level protocols, starting all the way down at the link layer. Session analysis handles application-layer protocols, such as HTTP, FTP, etc. File analysis dissects the content of files transferred over sessions. The event engine provides a plugin architecture for adding any of these from outside of the core Zeek code base, allowing to expand Zeek’s capabilities as needed.

Semantics related to the events are derived by Zeek’s second main component, the script interpreter, which executes a set of event handlers written in Zeek’s custom scripting language. These scripts can express a site’s security policy, such as what actions to take when the monitor detects different types of activity.

More generally scripts can derive any desired properties and statistics from the input traffic. In fact, all of Zeek’s default output comes from scripts included in the distribution. Zeek’s language comes with extensive domain-specific types and support functionality. Crucially, Zeek’s language allows scripts to maintain state over time, enabling them to track and correlate the evolution of what they observe across connection and host boundaries. Zeek scripts can generate real-time alerts and also execute arbitrary external programs on demand. One might use this functionality to trigger an active response to an attack.